Interannual changes in trophic niche of walleye pollock, Gadus chalcogrammus inhabiting middle eastern coast of Korea: Influences of prey availabilities derived from oceanographic changes

Abstract

Influences of oceanographic changes on dietary choice and trophic niche for walleye pollock, Gadus chalcogramma inhabiting benthic environment in the East Sea off Korean coast were examined based on stomach contents and stable isotope analyses. Walleye pollock was shown to be bottom feeding carnivores that consumed mainly benthic crustaceans, cephalopods and teleosts Spatio-temporal dietary pattern analyses based on stomach content (SCA) and stable isotope (SIA) analyses revealed significant differences in diets and trophic niche in relation to year and water depth. During 2016, both carid shrimps and cephalopods were abundant in the diets of walleye pollock, with the contribution of the latter prey taxa decreased as water depth. Euphausiids particularly dominated the diets of individuals inhabiting shallower depth during 2017, whereas the contributions of carid shrimps and teleosts increased in the diets of walleye pollock inhabiting deeper depth. In terms of stable isotopes, carbon stable isotope value (d13C) was slightly higher at deeper habitat and during 2017. While nitrogen stable isotope (d15N) value was significantly enriched during 2016 than 2017, but there was no such pattern between shallower and deeper depth during both 2016 and 2017. Comprehensive analysis of SCA and SIA for walleye pollock showed that increased contribution of euphausiids during 2017 resulted in distinct decrease of trophic niche (i.e., trophic level) compared to co-occurring higher trophic level predators. Such interannual patterns of dietary choice and trophic niche can be linked to the changes in water temperature of the walleye pollock habitat. Compared to 2016, a decrease of relative water temperature at intermediate water column (i.e., 100-300 m) in 2017 may have caused increases of occurrence of euphausiids, which have resulted in an increase in the consumption of lower trophic level prey items (i.e., euphausiids) by walleye pollock and consequently, a decrease in relative trophic niche.